Diarrheal diseases constitute the greatest causes of mortality on a global scale. The northeast of Brazil has one of the highest rates of diarrhea morbidity and mortality in the world. We and others have documented the importance of the parasite Cryptosporidium in this clinical setting. However, many aspects of the epidemiology, the role of humoral immunity or breastfeeding in preventing infection, and the environmental sources for this infection have not yet been studied in detail. Therefore, we have chosen to prospectively study a newborn cohort in two urban favelas (slums) in Fortaleza, Brazil to begin to address these issues. The specific aims of this proposal are: 1) To examine the epidemiology of cryptosporidiosis in a newborn cohort from an urban favela followed until ages 1 to 4. Using prospective weekly surveillance, episodes of diarrhea illness will be identified. The clinical features of the illnesses, patterns of infant feeding, intrafamilial spread of infection, potential environmental sources for the parasite, and serologic response to infection will be studied; 2) To determine if the colostrum and/or breast milk of the mothers of the infants enrolled in the cohort study contain IgA and/or IgG anticryptosporidial antibodies. The presence of breast milk antibodies to Cryptosporidium in this population will be compared to Brazilian mothers of higher socioeconomic status and to mothers delivering at the University of Virginia; 3) To identify if the water resources of the favela contain the oocysts of Cryptosporidium; and 4) To determine the prevalence of serum anticryptosporidial IgG antibodies over a 2 year period in a rural population of children. The surveillance system described in this project represents a collaborative effort with Project 1 and Specific Aim 4 of this proposal is being done in collaboration with Project 5 of this ICIDR proposal. We anticipate that these studies will help to clarify the epidemiology of Cryptosporidium infection in a tropical, developing region. The role of breastfeeding and humoral immunity in protecting against infection, and the potential environmental sources of this parasite. As a result of this work, prevention and therapeutic strategies may be devised for a presently untreatable parasitic disease.